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<a name="Implementation_002ddefined-behavior"></a>
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<a name="Implementation_002ddefined-behavior-1"></a>
<h3 class="section">11.1 Implementation-defined behavior</h3>
<a name="index-implementation_002ddefined-behavior"></a>

<p>This is how CPP behaves in all the cases which the C standard
describes as <em>implementation-defined</em>.  This term means that the
implementation is free to do what it likes, but must document its choice
and stick to it.
</p>
<ul>
<li> The mapping of physical source file multi-byte characters to the
execution character set.

<p>The input character set can be specified using the
<samp>-finput-charset</samp> option, while the execution character set may
be controlled using the <samp>-fexec-charset</samp> and
<samp>-fwide-exec-charset</samp> options.
</p>
</li><li> Identifier characters.
<a name="Identifier-characters"></a>
<p>The C and C++ standards allow identifiers to be composed of &lsquo;<samp>_</samp>&rsquo;
and the alphanumeric characters.  C++ also allows universal character
names.  C99 and later C standards permit both universal character
names and implementation-defined characters.  In both C and C++ modes,
GCC accepts in identifiers exactly those extended characters that
correspond to universal character names permitted by the chosen
standard.
</p>
<p>GCC allows the &lsquo;<samp>$</samp>&rsquo; character in identifiers as an extension for
most targets.  This is true regardless of the <samp>std=</samp> switch,
since this extension cannot conflict with standards-conforming
programs.  When preprocessing assembler, however, dollars are not
identifier characters by default.
</p>
<p>Currently the targets that by default do not permit &lsquo;<samp>$</samp>&rsquo; are AVR,
IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX
operating system.
</p>
<p>You can override the default with <samp>-fdollars-in-identifiers</samp> or
<samp>fno-dollars-in-identifiers</samp>.  See <a href="Invocation.html#fdollars_002din_002didentifiers">fdollars-in-identifiers</a>.
</p>
</li><li> Non-empty sequences of whitespace characters.

<p>In textual output, each whitespace sequence is collapsed to a single
space.  For aesthetic reasons, the first token on each non-directive
line of output is preceded with sufficient spaces that it appears in the
same column as it did in the original source file.
</p>
</li><li> The numeric value of character constants in preprocessor expressions.

<p>The preprocessor and compiler interpret character constants in the
same way; i.e. escape sequences such as &lsquo;<samp>\a</samp>&rsquo; are given the
values they would have on the target machine.
</p>
<p>The compiler evaluates a multi-character character constant a character
at a time, shifting the previous value left by the number of bits per
target character, and then or-ing in the bit-pattern of the new
character truncated to the width of a target character.  The final
bit-pattern is given type <code>int</code>, and is therefore signed,
regardless of whether single characters are signed or not.
If there are more
characters in the constant than would fit in the target <code>int</code> the
compiler issues a warning, and the excess leading characters are
ignored.
</p>
<p>For example, <code>'ab'</code> for a target with an 8-bit <code>char</code> would be
interpreted as &lsquo;<samp>(int)&nbsp;((unsigned&nbsp;char)&nbsp;'a'&nbsp;*&nbsp;256&nbsp;+&nbsp;(unsigned&nbsp;char)&nbsp;'b')</samp>&rsquo;<!-- /@w -->, and <code>'\234a'</code> as &lsquo;<samp>(int)&nbsp;((unsigned&nbsp;char)&nbsp;'\234'&nbsp;*&nbsp;256&nbsp;+&nbsp;(unsigned&nbsp;char)&nbsp;'a')</samp>&rsquo;<!-- /@w -->.
</p>
</li><li> Source file inclusion.

<p>For a discussion on how the preprocessor locates header files,
<a href="Include-Operation.html#Include-Operation">Include Operation</a>.
</p>
</li><li> Interpretation of the filename resulting from a macro-expanded
&lsquo;<samp>#include</samp>&rsquo; directive.

<p>See <a href="Computed-Includes.html#Computed-Includes">Computed Includes</a>.
</p>
</li><li> Treatment of a &lsquo;<samp>#pragma</samp>&rsquo; directive that after macro-expansion
results in a standard pragma.

<p>No macro expansion occurs on any &lsquo;<samp>#pragma</samp>&rsquo; directive line, so the
question does not arise.
</p>
<p>Note that GCC does not yet implement any of the standard
pragmas.
</p>
</li></ul>

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